1. Field of the Invention
The present invention relates to an apparatus for obtaining an ultrasonic image, which three-dimensionally transmits an ultrasonic wave into a subject and receives the reflected wave from the subject, thereby obtaining diagnosis information within the subject, and more particularly, to an apparatus for obtaining an ultrasonic image, which performs scanning by using an electrocardiographic signal (ECG signal).
2. Description of the Related Art
Recently, a so-called two-dimensional ultrasonic probe is being developed, in which ultrasonic transducers are two-dimensionally arranged so as to three-dimensionally transmit and receive an ultrasonic beam into and from a subject.
As shown in FIG. 1, an apparatus for obtaining an ultrasonic image, provided with the two-dimensional ultrasonic probe, can transmit and receive an ultrasonic beam three-dimensionally. Therefore, the apparatus can scan the entire region of a region of interest (ROI) in a short time in comparison with an apparatus for obtaining an ultrasonic image, which is provided with a one-dimensional ultrasonic probe having ultrasonic transducers arranged one-dimensionally. Particularly, the availability thereof is noticeable in circulatory organs in which a heartbeat is present.
The volume data obtained by three-dimensionally scanning is subjected to image processing, such as volume rendering (hereinafter, referred to as ‘VR processing’) or MPR (Multiplannar Reconstruction) processing. Then, three-dimensional image data or image data in an arbitrary cross-section is generated.
However, when scanning is performed by the two-dimensional ultrasonic probe, an amount of data generated per unit of time rapidly increases, compared with when scanning is performed by the one-dimensional ultrasonic probe. For example, when scanning a wide range of 60°×60°, the two-dimensional ultrasonic probe needs to be provided with a beam former, of which the parallel simultaneous reception number of ultrasonic beam (reception beam) is about 16. Accordingly, such a hardware is needed, that can process data generated by the parallel simultaneous reception number of about 16. Such enlargement of hardware significantly increases a cost of the apparatus for obtaining an ultrasonic image, which becomes a large obstacle to spread of the apparatus.
In the conventional apparatus for obtaining an ultrasonic image, a hardware of which the parallel simultaneous reception number of ultrasonic beam (reception beam) is about four is adopted to improve the ratio of cost to effect. Therefore, in order to obtain an image having a quality available for diagnosis without damaging a real-time property, a scanning range S shown in FIG. 1 needs to be narrowed into a scanning range S shown in FIG. 2, when the scanning is performed by using the apparatus for obtaining an ultrasonic image.
As a method of overcoming a constraint of transmission and reception caused by the scale of hardware, a method has been proposed, in which the entire scanning range S is divided into a plurality of regions to perform scanning (refer to U.S. Pat. No. 6,544,175). Hereinafter, the method will be described with reference to FIGS. 3A and 3B.
As shown in FIG. 3A, the apparatus for obtaining an ultrasonic image according to the related art divides the entire scanning range S into a plurality of regions. In the example shown in FIG. 3A, the apparatus for obtaining an ultrasonic according to the related art divides the entire scanning range S into four regions A to D. Hereinafter, the respective divided regions A to D are referred to as sub-volumes. In the example shown in FIG. 3A, four sub-volumes are respectively set to sub-volumes A to D. The apparatus for obtaining an ultrasonic image according to the related art divides the entire scanning range S, so that the sub-volumes A to D are lined up in order of A to D. Further, the apparatus for obtaining an ultrasonic image, of which the parallel simultaneous reception number of ultrasonic beam (reception beam) is small, scans an ultrasonic beam by the sub-volume, as shown in FIG. 3B.
The scanning of the respective sub-volumes, performed by the apparatus for obtaining an ultrasonic image according to the related art, will be described with reference to FIGS. 4A to 4D. FIG. 4A is a schematic view showing a scanning range of the apparatus for obtaining an ultrasonic image according to the related art. FIGS. 4B to 4D are schematic views illustrating the scanning range and scanning direction of the apparatus for obtaining an ultrasonic image according to the related art, and are diagrams (top views) seen from the ultrasonic probe.
As shown in FIGS. 4A and 4B, the apparatus for obtaining an ultrasonic image according to the related art scans an ultrasonic beam in a main scanning direction X and scans an ultrasonic beam in a sub-scanning direction Y orthogonal to the main scanning direction X, thereby scanning the entire range of the sub-volume A. Further, the apparatus for obtaining an ultrasonic image according to the related art scans the sub-volume A several times during one heartbeat and scans the sub-volume B at the next heartbeat. Similarly, the apparatus for obtaining an ultrasonic image scans the sub-volumes C and D.
FIGS. 4C and 4D show the sub-scanning direction in the sub-volumes A and B. As shown in FIG. 4C, the apparatus for obtaining an ultrasonic image according to the related art scans an ultrasonic beam in the sub-scanning direction Y (from the left side to the right side in the drawing), thereby scanning the sub-volume A. In the sub-volume A, the apparatus for obtaining an ultrasonic image according to the related art scans an ultrasonic beam toward the boundary with the sub-volume B.
As shown in FIG. 4D, the apparatus for obtaining an ultrasonic image according to the related art scans an ultrasonic beam in the same sub-scanning direction Y (from the left side to the right side in the drawing) as the sub-volume A, thereby scanning the sub-volume B. In the sub-volume B, the apparatus for obtaining an ultrasonic image according to the related art starts scanning from the boundary with the sub-volume A so as to scan an ultrasonic beam toward the boundary with the sub-volume C.
On the sub-volumes C and D, the apparatus for obtaining an ultrasonic image according to the related art also scans an ultrasonic beam in the sub-scanning direction Y (from the left side to the right side in the drawing), thereby scanning the sub-volumes C and D. Further, the apparatus for obtaining an ultrasonic image according to the related art combines the scan data obtained by scanning the respective sub-volumes so as to generate the scan data of the entire scanning range.
However, since the apparatus for obtaining an ultrasonic image according to the related art scans an ultrasonic beam in the same sub-scanning direction in the respective sub-volumes, the following problem occurs. The problem will be described with reference to FIG. 5. FIG. 5 is a schematic view illustrating the scanning range and scanning direction of the apparatus for obtaining an ultrasonic image according to the related art, and is a diagram (top view) seen from the ultrasonic probe.
The apparatus for obtaining an ultrasonic image according to the related art combines scan data, obtained at a different heartbeat and obtained at the same time phase, so as to generate one volume data corresponding to the entire scanning range.
For example, the apparatus for obtaining an ultrasonic image according to the related art scans the sub-volumes A to D between time phases t0 and t1 so as to obtain scan data A0 to D0. Then, the apparatus combines the scan data A0 to D0 so as to generate the scan data of the entire range of a region of interest between the time phases t0 and t1.
FIG. 5 shows a portion of the scan data generated in such a manner. For a simple explanation, only the scan data A0 and B0 are shown in FIG. 5. On both the sub-volumes A and B, the apparatus scans an ultrasonic beam in the same scanning directions (the main scanning direction X and the sub-scanning direction Y), thereby obtaining the scan data A0 and B0.
Here, attention is paid to the vicinity of the boundary L between the sub-volumes A and B. In the sub-volume A, one line of scan data obtained in a scanning range (the vicinity of the boundary L with the sub-volume B) of the right end thereof is data obtained between time phases (t1−δt) and t1. Meanwhile, in the sub-volume B, one line of scan data obtained in a scanning range (the vicinity of the boundary L with the sub-volume A) of the left end thereof is data obtained between time phases t0 and (t1+δt). Here, δt means a time which is required for scanning one line at the time of radiating an ultrasonic beam in the main scanning direction X. Accordingly, in the vicinities of the boundary L between the sub-volumes A and B, a difference of about Δt is present in the time phases where the scan data are obtained.
For example, when the sub-volume is scanned 20 times during one heartbeat and a time of one heartbeat is set to one second, a scanning time Δt required for performing scanning one time becomes 0.05 second (Δt= 1/20). Therefore, in the vicinities of the boundary L between the sub-volumes A and B, the time phase difference of about 0.05 second occurs. That is, in the vicinities of the boundary L between the sub-volumes A and B, the scan data are obtained at different time phases (Δt=0.05).
The time phase difference (about 0.05 second) is such a value that cannot be ignored in a portion, such as a valve or core wall of the heart, in which the motion is severe. When the scanning according to the related art is performed on a part, in which the motion is severe, so as to generate a three-dimensional image or MPR image based on the scan data obtained by the scanning, a streaky artifact is generated in a position corresponding to the boundary between the respective sub-volumes.
The artifact will be described with reference to FIGS. 6A and 6B. FIG. 6A is a schematic view showing a scanning range of the apparatus for obtaining an ultrasonic image according to the related art. FIG. 6B is a diagram showing an image obtained by the scanning performed by the apparatus for obtaining an ultrasonic image according to the related art.
As shown in FIG. 6A, the apparatus for obtaining an ultrasonic image according to the related art divides a scanning range into sub-volumes A to D, the scanning range including a diagnosis portion 100 such as the heart. Then, the apparatus scans the respective sub-volumes. Further, the apparatus for obtaining an ultrasonic image according to the related art combines the scan data obtained at the same time phase so as to generate a three-dimensional image or MPR image through the volume rendering or MPR processing.
FIG. 6B shows an image obtained by performing the rendering process on the obtained scan data, in a state where an observation direction P shown in FIG. 6A is set to a direction of line of sight. As described above, the time phase difference occurs in the vicinities of the boundary between the sub-volumes. Therefore, in an image 101 of the diagnosis portion 100, a vertically-running streaky artifact 102 is generated in a position corresponding to the boundary between the sub-volumes.